(AFP) - US troops swooped in on helicopters to evacuate embassy staff from Sudan's battle-torn capital, President Joe Biden said Sunday, as other nations sought to help their citizens flee deadly fighting between rival generals.

France and Turkey on Sunday also launched evacuation operations from the troubled northeast African nation, where ongoing fighting has entered its second week.

Ferocious battles between the Sudanese army and a paramilitary group -- which has seen fighting with tanks in densely populated Khartoum and air strikes launched by fighter jets -- have killed more than 400 people and left thousands wounded.

Biden, who said the US military "conducted an operation" to extract US government personnel, condemned the violence, saying "it's unconscionable and it must stop".

Just over 100 US special operations troops took part in the rescue to extract fewer than 100 people, which saw three Chinook helicopters fly from Djibouti, staying on the ground in Khartoum for less than an hour.

France's foreign ministry said Sunday a "rapid evacuation operation" had begun, and that European citizens and those from "allied partner countries" would also be assisted, without giving further details.

Fighting continued Sunday with the crackle of automatic gunfire echoing across Khartoum and Sudanese military aircraft roaring overhead, witnesses said.

Turkey began rescue operations at dawn via road from the southern city of Wad Medani, but plans were postponed from one site in Khartoum after "explosions" near a mosque designated as the assembly area, the embassy said on Twitter.

Long lines of United Nations vehicles and buses were seen leaving Khartoum heading east to Port Sudan on the Red Sea, 850 kilometres (530 miles) by road.

- Rush to escape -

Frightened residents, many low on water, food and other essentials, have huddled inside their homes, only venturing out during rare lulls in the capital, where buildings have been gutted, streets are littered with charred tanks, and smoke has been rising from shops set on fire.

Heavy fighting broke out on April 15 between forces loyal to army chief Abdel Fattah al-Burhan and his deputy turned rival Mohamed Hamdan Daglo, who commands the powerful paramilitary Rapid Support Forces (RSF).

The former allies seized power in a 2021 coup but later fell out in a bitter power struggle.

Daglo's RSF emerged from the Janjaweed fighters unleashed in Darfur by former strongman leader Omar al-Bashir, where they were accused of war crimes.

Pope Francis on Sunday renewed calls "for the violence to stop as quickly as possible and for dialogue to resume."

Multiple truces have been agreed and ignored.

Khartoum's airport has been the site of heavy fighting with aircraft destroyed on the runway, and is under the control of the RSF.

US Under Secretary of State John Bass said that the RSF "cooperated to the extent that they did not fire on our service members", warning any wider effort to evacuate thousands of other American citizens was unlikely in the coming days.

More than 150 people from various nations reached the safety of Saudi Arabia after naval forces launched a rescue across the Red Sea on Saturday, collecting both Saudi citizens and nationals from 12 other countries from Port Sudan.

Other foreign countries have said they are preparing for the potential evacuation of thousands more of their nationals.

But the scramble by foreigners to escape has sparked worry among Sudanese of what will happen when diplomats who could act as potential mediators have gone.

"Pushing for safe passages to evacuate internationals without simultaneously pushing to end the war will be terrible", said researcher Hamid Khalafallah.

"International actors will have less impact once they're out of country," he said, adding in a message to foreign nations: "Do all you can to leave safely, but don't leave the Sudanese people behind unprotected."

- 'Living in darkness' -

In Khartoum, a city of five million, the conflict has left terrified civilians sheltering inside their homes, with power largely off amid sweltering heat and the internet out for most.

"We were living in darkness...first we didn't have water and then we didn't have power," Khartoum resident Awad Ahmad Sherif said. "We ask God for our safety."

Fighting has broken out elsewhere across Sudan, Africa's third biggest nation and roughly three times the size of France.

Battles have raged in Darfur, where aid group Doctors Without Borders (MSF) in the city of El Fasher said their medics had been "overwhelmed" by the number of patients with gunshot wounds, many of them children.

The UN World Health Organization said more than 420 people had been killed and over 3,700 wounded in the fighting across Sudan, but the actual death toll is thought to be higher.

Some hospitals have been shelled in fighting and others looted, with more than two-thirds of hospitals in Khartoum and neighbouring states "out of service", the doctors' union said.

Burhan and Daglo's dispute centred on the planned integration of the RSF into the regular army, a key condition for a deal aimed at restoring Sudan's democratic transition after the military toppled Bashir in April 2019 following mass citizen protests.

© Agence France-Presse